This disclosure relates to the use of molecular genetic technology involving LysM receptor kinase family genes and the expression or nonexpression thereof to modulate plant defense responses, especially against fungal pathogens.
Fungal disease causes significant agricultural losses in the United States and other parts of the world. Control of these pathogens is particularly difficult, often requiring treatment of entire fields with biocidal compounds. Although effective, increasing concern about the environmental and economic costs of such treatments require the need for alternative control methods.
Phakopsora pachyrhizi is a fungus that causes a rust disease of soybean (Glycine max), also known as Asian Soybean Rust. The pathogen has spread from Asia to all other soybean production regions in the world, and is reported to have arrived in the United States in the fall of 2004. At present, there is no known durable resistance available in any soybean varieties. Uromyces appendiculatus is a fungus that causes rust on bean (Phaseolus vulgaris). Breeders are working to identify genes in bean that can be manipulated for rust resistance. United States soybean producers have anticipated the arrival of P. pachyrhizi, the fungus that causes soybean rust, since its reported occurrence in Brazil. The arrival of P. pachyrhizi in the U.S. in late 2004 ended the anticipation, and farmers must now respond to the potential annual occurrence of this new disease. Farmer concerns have been based on reports of losses ranging from 10 to 80% in other regions of the world when control measures were not successfully implemented.
As rust-inducing fungi, U. appendiculatus and P. pachyrhizi belong to the order Uredinales, within the class Basidiomycetes. U. appendiculatus produces five spore stages on a single host plant. P. pachyrhizi reproduces predominantly by uredospores on a single host plant. Uredospores are responsible for rapid spread of the fungus. P. pachyrhizi can infect dozens of legume species, in addition to soybean.
Uredospores of U. appendiculatus penetrate through foliar stomatal openings. P. pachyrhizi differs in that germinated uredospores penetrate directly through the leaf epidermal cell layer. Typically, a uredospore that lands on a leaf surface germinates to produce an infection pad (appressorium) that adheres to the surface. In both species, the appressorium produces a hyphal peg that penetrates the plant. After penetration, each fungus develops thread-like structures (hyphae) that grow inter-cellularly through leaf tissues. The hyphae enter host cells without killing them. There, they produce spherical structures (haustoria) that extract nutrients from the living leaf cells. Soon after infection each fungus forms uredia that produce additional spores.
Soybean producers are particularly concerned because no durable, natural resistance to rust has been discovered after testing more than 18,000 soybean varieties. The pathogen, P. pachyrhizi can potentially infect any cultivar produced. In anticipation of the arrival of the rust pathogen, a great deal of research has been conducted to identify effective fungicides, and emergency governmental clearance for application to soybean has been obtained. Traditional screening and breeding methods have identified no major resistance genes to the aforementioned pathogens, and particularly in the case of U. appendiculatus and P. pachyrhizi. 
Fungicides will likely be the front-line of defense against these and other fungal pathogens for many years until new resistance genes or other forms of resistance are identified. Fungicides have not traditionally been used in most soybean production. Consequently, there is limited information concerning the costs of this disease management practice and its likely economic viability. Widespread use of fungicides may also raise environmental concerns. These concerns have led to variable estimates of the acreage in Missouri and other states that may be shifted from soybean to alternative crops.
To protect soybean farmers and to ensure that soybean production meets increasing market demand, it is imperative that alternatives to fungicides be developed as rapidly as possible. Biotechnology-based approaches for defense against plant diseases are more preferable due to the minimal use of chemicals and the relative ease of deployment.
Chitin is a polymer of N-acetyl-D-glucosamine, found in fungal cell walls, insect exoskeletons, and crustacean shells. It has been hypothesized that plant chitinases can degrade chitin in the fungal cell walls to directly affect the viability of the invading fungal pathogen and to release short fragments (chitooligosaccharides) that can act as a general elicitor of plant innate immunity pathways. See e.g., Shibuya et al., 2001 and Stacey et al., 1997. In support of the above hypothesis, purified chitooligosaccharides have been shown to induce various defense responses in plants or cultured cells, such as induction of defense related genes and synthesis of phytoalexin. Shibuya et al., 2001 and Ramonell et al., 2005.
More particularly, chitooligosaccharides have been shown to induce a large number of genes (including many defense-related genes), and mutations in selected chitooligosaccharide-responsive genes or chitin-responsive genes (CRGs) have been shown to increase the susceptibility of a plant to certain fungal pathogens. See Ramonell et al., 2002; and Ramonell et al., 2005. Taken together, these studies suggest that plants possess a specific system to recognize chitooligosaccharides which, in turn, activate defense genes. See generally, Day et al., 2001; Zhang et al., 2002; Wan et al., 2004; Kaku et al., 2006; and Libault et al., 2007.
Previous work has reported chitin recognition in rice and legumes. Stacey, G. and N. Shibuya, Plant and Soil 194: 161-169 (1997) The ability of Arabidopsis thaliana to recognize and respond to chitin has also been reported. A variety of genes have been shown to respond to chitin treatment. See e.g., Ramonell et al. Microarray analysis of chitin elicitation in Arabidopsis thaliana. Mol. Plant Pathol. 3 (1): 301-311 (2002) and Zhang et al., Characterization of Early, Chitin-Induced Gene Expression in Arabidopsis Mol. Plant-Microbe Int. 15: 963-970 (2002) and Wan et al., Activation of a potential mitogen-activated protein kinase pathway in Arabidopsis by chitin. Mol. Plant Pathol. 5(1): 125-135 (2004).
More specifically, chitin binding sites or proteins have been previously identified in membrane preparations of a variety of plant cells. Day et al., 2001; Ito et al., 1997; and Okada et al, 2002. More recently, a LysM domain-containing protein (CEBiP) has been shown to be involved in the binding and recognition of chitooligosaccharides in rice. Kaku et al., 2006. The LysM motif was originally identified in bacterial enzymes that degrade cell wall component peptidoglycan, which is structurally similar to chitin. Joris, 1992. Since CEBiP lacks a significant intracellular domain, it likely functions as part of a chitin receptor complex. Kaku et al., 2006. However, no such chitin receptor complexes have been identified.